The Crypto Gene was found by serendipity in a screening of a cDNA library from human teratocarcinoma cells. The gene is expressed only in undifferentiated teratocarcinoma cells and shuts off on exposure of the cells to retinoic acid which induces differentiation. The gene is expressed in mouse teratocarcinoma cells and behaves similarly. The sequence of the gene looks remarkably like the growth factor EGF, but is clearly distinct from that factor in its expression. When placed under the control of an RSV promoter, crypto gene expression results in the transformation of certain but not all cell types. Crypto gene expression has been found only in certain basal cell carcinomas. It was also found in an expression library made from early blastula rat embryos. On the other hand 11 day embryos showed no detectable levels of expression. A baculovirus expression vector has been made.